In recent years it has become more and more common to install the rotor blades of a wind turbine individually, i. e. one at a time, to the hub which is already mounted on the wind turbine tower. This so-called single blade installation procedure provides many benefits, especially when wind turbines are installed in offshore parks, mountainous or forest areas, or other sites where space is limited. Moreover, maintenance is more economic when a single blade can be replaced right on the spot instead of having to take down the entire rotor arrangement first, which requires at least one rather large crane.
However, it has to be considered that usually the drivetrain of a wind turbine can only be rotated when either none or all rotor blades are installed. This means that after installation of the first blade, the installation positions for the further rotor blades are fixed. Hereinbelow, any blade installation position is to be referred to the axis of rotation of the hub as illustrated in FIG. 1, the 0° position being the position in which the tip of the respective blade has the highest position possible. For example, if the first blade of a three blade rotor is installed in the 180° position, the other two blades have to be installed in the 60° position and in the 300° position, respectively.
Another important aspect of the single blade installation is the number of cranes used for lifting and tilting the blades towards their installation positions. While it is easier to handle a blade with two separate cranes at the same time, this method is difficult to implement offshore or at sites where space is limited and is therefore not economic in such areas. For that reason the single blade installation method would be preferred if there were suitable tools available to handle a blade with a single crane.
Several arrangements for handling long and heavy objects like wind turbine rotor blades are known in the prior art. For example, EP 2 623 768 A1 shows a lifting frame for lifting and tilting a wind turbine rotor blade with a single crane. In one embodiment a main frame, which is fixed to the rotor blade, can be tilted relative to a sub-frame using a rope or the like. By driving a tensioning winch, a rope section running from a pulley at the frame end facing the blade tip to a crane hook can be shortened, while at the same time a rope section running from a pulley at the frame end facing the blade root to the crane hook is lengthened. By this measure the main frame is aligned to the sub-frame, thus allowing the mounting of the rotor blade to the rotor hub of the wind turbine in a 60° or 300° position. This construction requires a main frame and a sub frame pivotably connected thereto. Due to the triangular rope path the tilting movement is limited. In particular, no vertical orientation of the rotor blade allowing a 180° installation can be reached.
In WO 2012/0062352 A1 a lifting beam for use in hoisting a wind turbine blade is shown. The lifting beam, which is suspended from a crane using two cables, includes a root manipulation system with a cable connected to a winch at one end and coupled to a sling carrying the root of the blade at the other end. By actuation of the winch, the height of the sling carrying the blade root can be adjusted, thereby changing the orientation of the blade. A similar tip manipulation system for raising or lowering the tip of the blade is also provided. However, despite the separate root and tip manipulation systems located at the very ends of the beam, this construction allows adjusting the orientation of the blade only within a small angular range. Rotating the blade to a 180° installation position is not possible.
U.S. Pat. No. 8 191 721 B2 relates to a wind turbine blade lifting system with a crane boom and a lifting device (frame) which can be connected to the wind turbine blade. The system further includes two individually controllable winches connected to two control wires and a bearing wire connected to a separate winch which is operated for lifting the lifting device with the turbine blade fixed thereto. The bearing wire is fixed to a central area of the lifting device while the control wires are fixed to the frame at distant ends thereof. The control wires run via pulleys mounted onto a sliding carriage which can be moved along the crane boom. The turbine blade is lifted into a substantially horizontal position while the sliding carriage with the pulleys follows the blade on its way upwards. By differently pre-tensioning the control wires, the horizontal orientation of the turbine blade is varied. Yet, rotating the turbine blade into a vertical position is not possible.
WO 03/100249 A1 shows a system for handling a wind turbine blade with a yoke-shaped gripping unit. The blade to be installed is positioned in the gripping unit close to its root end. The gripping unit, which includes a lever arm with a weight at its free end, is suspended with three crane wires. The first wire is connected to the free end of the lever arm at the back of the gripping unit, and to the crane on a crane bar above the blade. The second wire is connected to the front of the gripping unit facing the root of the blade, and at a different position on the same crane bar. The third wire is also connected to the free end of the lever arm, and to the main body of the crane. With the crane motor and the first and second wires it is possible to rotate the blade from a horizontal position into a vertical position. During lifting of the blade the third wire and the lever arm with the weight facilitate controlling the blade. This construction is only configured for a 180° blade installation. The specific geometry of the crane bar and the free arm of the gripping unit ensure that neither the crane nor any wire interferes with the turbine hub.
From WO 2012/095112 A1 a tool for handling wind turbine blades is known, which comprises a frame and a connection arrangement for connecting the frame to a crane wire. The frame includes two sets of gripping organs operated with hydraulically driven actuators for engagement of the blade. With a hydraulic powered yaw between the connection arrangement and the frame the blade can be turned from a substantially horizontal orientation to practically any preferred position during lifting and mounting of the blade on the blade anchoring. However, the construction is rather complex and has several drawbacks regarding the practical use. First, the hydraulic system, including hydraulic actuators, an oil supply system and a number of hydraulic lines, takes up significant installation space in the tool and requires extensive maintenance. Several sub-units of the hydraulic systems need to be matched to each other and have to be available redundantly. Further, the bearing of the tool's rotating arm for tilting the blade about its center of gravity is quite complex. The hydraulic and electrical supply of the gripping organs, which has to be ensured permanently, requires a technically difficult configuration. Eventually, the supply equipment and part of the counterweight are accommodated in a container at the rear of the tool. Since the container faces the crane boom during lifting, the safety clearance between the tool holding the blade and the crane boom is reduced, thus limiting operability under severe wind conditions.